The present invention relates to data communication receivers and, more particularly, to receivers that convert an incoming analog signal into digital data. Data communication receivers of this type are frequently used in telephone networks, which include hardware for the transmission of voiceband signals.
A growing number of businesses, industries and home computer users have come to rely on the ability to quickly move data from one point to another. Many of these users view the speed of the data transmission as critical to use of the transmission medium for communication. In addition, they have come to expect quality and reliability in the data transmission.
Most of this data traffic currently is carried over voiceband telephone networks. These telephone networks typically can carry signals that range in frequency from approximately 300 Hz to 3400 Hz, which, not coincidentally, roughly corresponds to the range of the human voice spectrum. Signals outside this range are sharply attenuated by the networks. This constraint on frequency range, or bandwidth, constrains the maximum attainable data rate.
Although the bulk of the long-distance and inter-office traffic on the telephone networks is carried digitally, many subscribers to the telephone networks are connected to the digital infrastructure by a two-wire analog line that is commonly referred to as a subscriber "loop." FIG. 1 shows a typical subscriber loop connection for the transmission of data to and from the switched digital telephone network 20. The basic elements of this subscriber loop connection are a modem 22 that is connected by a two-wire analog line 24 to a local switch 26, which terminates the switched digital telephone network 20.
The modem 22 is typically located at the subscriber's premises and includes a receiver 28 and a transmitter 30. As shown in FIG. 1, the receiver 28 and the transmitter 30 are coupled to the analog line 24 by a hybrid 32. The transmitter 30 converts input digital data 34 into analog signals that are passed through the hybrid 32 and transmitted over the analog line 24 to the local switch 26. Likewise, the receiver 28 converts input analog signals, which pass from the analog line 24 through the hybrid 32, into digital data 36.
At the local switch 26 end of the subscriber loop, analog signals from the line 24 are directed through a hybrid 38 to an analog-to-digital converter 40. The analog-to-digital converter 40 samples the analog signals converting them into a digital data stream for transmission through the switched digital telephone network 20. For transmission in the opposite direction, a digital data stream is applied from the digital telephone network 20 to a digital-to-analog converter 42. The digital-to-analog converter 42 converts the data stream into analog signals that are passed through the hybrid 38 to the analog line 24, for transmission to the appropriate subscriber.
Various standards have been adopted throughout the world for the analog-to-digital and digital-to-analog conversions. The United States, for example, uses a conversion scheme in which the analog-to-digital converter in the local switch samples the analog signals at the rate of 8000 samples per second and maps the samples into one of 255 possible distinct codewords. The 255 codewords correspond to quantization levels defined by a non-linear mapping rule called the .mu.-law companding rule, which is the Pulse Code Modulation ("PCM") voice coding and companding standard in North America and Japan. The codeword chosen for each sample corresponds to the quantization level that is closest to the voltage of the analog sample. The digital-to-analog converter in the local switch performs the inverse of this mapping, converting codewords into analog voltage signals.
The codewords utilized by the switched digital telephone network are typically eight bit codewords. FIG. 2 shows a bit allocation map for a .mu.-law codeword. In the eight bit codeword, the most significant bit, b.sub.7, is a sign bit. The next three bits, b.sub.6 through b.sub.4, identify one of eight segments in the claw quantization characteristic. The last four bits, b.sub.3 through b.sub.0, identify one of sixteen steps within that segment.
At the modem end of the subscriber loop, it is the function of the modem receiver to recover digital data from the received analog signal. Modem receivers, such as the receiver 28 in FIG. 1, typically include an adaptive equalizer, which is a digital signal processing device that dynamically adjusts the response of a modem's receiver. An adaptive equalizer may compensate for deleterious effects of the subscriber loops, which effects vary from connection to connection, as well as with time.
Devices are known that include adaptive equalizers. For example, U.S. Pat. No. 5,528,625 issued to Ayanoglu et al. shows a high speed modem with an equalization arrangement. The receiver component of the modem includes an analog-to-digital converter having an output that is coupled to six parallel receiver equalizers. Each of the receiver equalizers produces a sample stream that is operated on by a corresponding slicer to produce six slicer output symbol streams. A decoder maps the symbol streams into a data output stream. Training of the receiver equalizers is aided by an adaptation service unit within the digital telephone network. The adaptation service unit causes a known training sequence to be transmitted to the receiver. Appropriate coefficient settings for the six parallel receiver equalizers are then determined from the difference between their outputs and the known training sequence.
A disadvantage of the Ayanoglu device is its use of an adaptation service unit within the digital telephone network. It is desirable to avoid the addition of such structures to the digital telephone network infrastructure.
A further disadvantage of the Ayanoglu device is its use of six parallel receiver equalizers. It is desirable to minimize receiver complexity.
Accordingly, it would be desirable to have an improved method and apparatus for adaptively equalizing a signal received from a remote transmitter.